Google has introduced the Android Market website (http://market.android.com/) that lets you browse and search for great apps right from your web browser. Previously, you could only access Android Market directly from your device. Now using the website, you can easily discover great new apps with a bigger, brighter interface. You can also send apps directly to your Android device with just a few clicks—no wires needed. You can also share apps with your friends through Twitter. And you can read and post app reviews directly to Android Market from the web or from your device.
From Google Mobile blog,

And what about all those apps you’ve already purchased and downloaded? Sign in to the website with your Google account and click “My Market Account” to see all the apps you’ve purchased or downloaded. It makes managing all your apps quite easy.

The Android Market website was officially unveiled during an event at Google headquarters where the company showcased Honeycomb (Android 3.0), their latest Android release built from the ground up for tablets. Honeycomb has a completely redesigned user interface, with more interactive notifications and widgets, improved multi-tasking, and the latest and greatest Google Mobile services optimized for tablets.